Preparation of CuCrO 2 Anisotropic Dela-fossite-Type Thin Film by Electrospinning on Glass Substrates

: Anisotropic CuCrO 2 thin ﬁlms were successfully prepared on glass substrate by the electrospinning method followed by annealing at 600 ◦ C. The directionality of the nanowires increased with increases in rotation speed. In addition, the structural formation and optoelectronic behavior of the CuCrO 2 thin ﬁlms have been studied by X-ray powder diffraction studies, transmission electron microscopy, ﬁeld emission scanning electron microscopy, and UV-vis spectroscopy. The X-ray diffraction studies revealed a delafossite properties of the CuCrO 2 . The electrical conductivity in the parallel and vertical directions of the ﬁlms produced at 2000–3000 rpm differed by two to three orders of magnitude. The optical transmittance properties of the CuCrO 2 thin ﬁlms were improved by increasing the rotation speed.


Introduction
Metal oxides are a class of materials with a wide range of characteristics that are becoming increasingly used in a variety of applications. Moreover, metal oxides have a high heat of formation and a huge band gap due to the strong metal-oxygen bond [1,2]. Transparent conducting oxides (TCO) are special semiconductors combining electrical conductivity and visible-range transparency in a single material [3]. They are used in a wide range of optoelectronic applications, such as photovoltaic cells, solar cells, flat panel displays, and gas sensors, and light-emitting diodes [4][5][6][7]. The popular TCOs currently used are n-type, of which In-doped SnO 2 and F-doped SnO 2 dominate the market for unique properties [8]. However, improvements to modern optoelectronic devices are based on p-n junctions, so the development of p-type materials is important. At present, p-type TCOs are generally less evolved and less used than n-type TCOs. The CuAlO 2 was first described in 1997 for a highly conductive p-type TCO material [9]. A series of p-type TCOs based on Cu-incorporating oxides, such as CuCrO 2 [10], CuFeO 2 [11], CuInO 2 [12], SrCu 2 O 2 [13], CuScO 2 , and CuGaO 2 [14] have been established as a result of material discovery efforts following the principle. Hence, the improvement of p-type TCO materials is a worthy endeavor.
The CuCrO 2 is p-type TCO [15,16], which can be prepared by different techniques, such as chemical vapor deposition (CVD) [17], pulsed laser deposition [18][19][20][21], sol-gel techniques [22,23], hydrothermal synthesis [24], electron beam evaporation [25], sputtering [26][27][28] and so on. However, electrospinning is more interesting because it is conducted at room temperature [29]. It is also a simple technique for nanowire preparation [30]. There are three key components of an electrospinning system: a high voltage power unit, a spinneret, and a collecting plate. A voltage is applied to the metal source to inject a charge of a certain polarity into a loaded polymer solution, which is then expelled into an opposite polarity collector [31,32]. A polymer gel solution retained by its surface tension at the end of a tube is subjected to an electric field in the electrospinning. A charge is caused by the applied electric field on the liquid top surface. Repulsive electrostatic charge produces a force that is directly opposite to the surface tension of the polymer. The repulsive electrical force surpasses the surface tension force when the applied electric field exceeds a critical value [33]. Eventually, from the tip of the tube, the charged polymer solution is expelled, forming a Taylor cone and becoming unstable. As a result, the jet is easily whipped between the capillary tip and the collector, resulting in solvent evaporation [34,35]. In this article, we describe the preparation of CuCrO 2 films by a simple electrospinning method and investigations of the prepared CuCrO 2 films to determine their morphological structures, electrical, and optical properties. The results indicated that the rotation speed and temperature influence the morphological structure of CuCrO 2 nanowires.

Instrumentation
Copper nitrate (Cu(NO 3 ) 2 ) and chromium (II) acetate were purchased from Aldrich and AVATOR. Ethanol and polyvinyl pyrrolidone (PVP) were purchased from Echo Chemical Co., Ltd. A study of field emission scanning electron microscopy (FE-SEM) was performed with a Hitachi S-470000. The material structure was identified with a JEOL 2100F transmission electron microscope (TEM). The elemental analysis (EDX) was performed with a HORIBA EMAX-ACT (model 51-ADD0009). The crystal structures were characterized with a GA-XRD PANalytical X'pert X-ray diffractometer (XRD). The optical transmission spectra were measured with a Shimadzu UV-2600 model spectrometer at a range of 200-1100 nm. The electrical conductance of the nanowires was measured with KEITHLEY 2634B two-point probe method at room temperature. To measure the electrical conductance, Al-doped ZnO (AZO) thin films were deposited by RF magnetron sputtering via scotch tape as a mask to form two contact electrodes, as shown in Figure 12.

CuCrO 2 Thin Film Preparation
The CuCrO 2 nanowire was prepared by a simple electrospinning method. The sol-gel method with CuCrO 2 precursor used for the film preparation is illustrated in Figure 1. Chromium acetate ((CH 3 CO 2 ) 7 Cr 3 (OH) 2 ) and cupric nitrate (Cu(NO 3 ) 2 ·3H 2 O) were combined with a molar ratio of 3:1, and the prepared composition was dissolved in ethanol (CH 3 CH 2 OH) to obtain 0.2 M of the metal source solution. At the same time, polyvinyl pyrrolidone (PVP) ((C 6 H 9 NO) n , average MW:1,300,000) (1.5 g) of and ethanol (8.5 mL) were applied to the metal source solution (10 mL) to form the main precursor. After effective stirring at 80 • C for 12 h, a sticky gel-like solution of CuCrO 2 /PVP precursor was achieved. The reaction precursor was put in a 5 mL syringe for the electrospinning process. The syringe was then clamped to a ring stand 5 cm above a stainless steel mesh that was grounded and bowl-shaped. A high-voltage power was attached to the precursor syringe's metal needle, which was linked to the syringe pump. The applied voltage was 16 kV, and the electrospinning precursor feeding rate was 0.12 mL/h. Anisotropic films were prepared using the high-speed spinning collector and the glass substrate was anchored on the roller. The CuCrO 2 nanowires were electro-spun onto the glass substrate for 2.5 h. Every sample was dried in an oven for 12 h at 80 • C after electrospinning. The dried CuCrO 2 precursor as spun fiber films on glass substrate were annealed in vacuum condition at 600 • C for 10 min to form CuCrO 2 films.
Every sample was dried in an oven for 12 h at 80 °C after electrospinning. The dried Cu-CrO2 precursor as spun fiber films on glass substrate were annealed in vacuum condition at 600 °C for 10 min to form CuCrO2 films.

XRD Analysis of CuCrO2 Thin Films
The X-ray diffraction analysis (XRD), used to identify the crystal structure of the Cu-CrO2 thin films prepared by electrospinning with 3000 rpm rotating speed on glass substrate after annealing, is shown in Figure 2. The XRD pattern exhibited three main peaks, corresponding to the (006), (012) and (110) planes, which coincided with the typical delafossite structure of CuCrO2 (JCPDF Card No. 39-0247). Moreover, based on the XRD pattern study, it was determined that the pure phase of CuCrO2 was obtained by annealing at 600 °C for 10 min.

XRD Analysis of CuCrO 2 Thin Films
The X-ray diffraction analysis (XRD), used to identify the crystal structure of the CuCrO 2 thin films prepared by electrospinning with 3000 rpm rotating speed on glass substrate after annealing, is shown in Figure 2. The XRD pattern exhibited three main peaks, corresponding to the (006), (012) and (110) planes, which coincided with the typical dela-fossite structure of CuCrO 2 (JCPDF Card No. 39-0247). Moreover, based on the XRD pattern study, it was determined that the pure phase of CuCrO 2 was obtained by annealing at 600 • C for 10 min.
Every sample was dried in an oven for 12 h at 80 °C after electrospinning. The dried Cu-CrO2 precursor as spun fiber films on glass substrate were annealed in vacuum condition at 600 °C for 10 min to form CuCrO2 films.

XRD Analysis of CuCrO2 Thin Films
The X-ray diffraction analysis (XRD), used to identify the crystal structure of the Cu-CrO2 thin films prepared by electrospinning with 3000 rpm rotating speed on glass substrate after annealing, is shown in Figure 2. The XRD pattern exhibited three main peaks, corresponding to the (006), (012) and (110) planes, which coincided with the typical delafossite structure of CuCrO2 (JCPDF Card No. 39-0247). Moreover, based on the XRD pattern study, it was determined that the pure phase of CuCrO2 was obtained by annealing at 600 °C for 10 min.   Figure 3 presents the optical microscope analysis of CuCrO 2 thin films deposited on glass substrate at different rotation speeds (100-3000 rpm). As shown in Figure 3A, slow spinning produced disordered nanowires. At a rotation speed of 500 and 1000 rpm, as shown in ( Figure 3B,C), the nanowires were still somewhat disordered. At 2000 rpm and above, the nanowire alignment had a more homogeneous direction, as shown in Figure 3D,E.

Electron Microscope Studies of Prepared CuCrO 2 Thin Films
Ceramics 2021, 4 FOR PEER REVIEW 4 Figure 3 presents the optical microscope analysis of CuCrO2 thin films deposited glass substrate at different rotation speeds (100-3000 rpm). As shown in Figure 3A, s spinning produced disordered nanowires. At a rotation speed of 500 and 1000 rpm shown in (Figure 3B,C), the nanowires were still somewhat disordered. At 2000 rpm above, the nanowire alignment had a more homogeneous direction, as shown in Figu D,E.    The CuCrO2 structure on glass substrate was identified by TEM. The TEM sam were processed with a focused ion beam (FIB) with a Pt protective coating. The sam were then cut in directions vertical and parallel to the spinning direction. Figures 6  and 7A,B present TEM images of the CuCrO2 thin films prepared by electrospinning w a rotating speed of 3000 rpm cut in the two directions. The parallel-direction TEM im clearly shows that the films were well attached to the AZO, and the vertical-direction T image exhibits that the films were deposited in layers on the AZO. The EDX analys Figures 6C and 7C presents the mapping regions corresponding to (D) Cu, (E) Cr, (F and (G) Zn elements. All results confirmed that the anisotropic CuCrO2 thin films w successfully prepared on the AZO by electrospinning.  The CuCrO 2 structure on glass substrate was identified by TEM. The TEM samples were processed with a focused ion beam (FIB) with a Pt protective coating. The samples were then cut in directions vertical and parallel to the spinning direction. Figure 6A,B and Figure 7A,B present TEM images of the CuCrO 2 thin films prepared by electrospinning with a rotating speed of 3000 rpm cut in the two directions. The parallel-direction TEM image clearly shows that the films were well attached to the AZO, and the vertical-direction TEM image exhibits that the films were deposited in layers on the AZO. The EDX analysis in Figures 6C and 7C presents the mapping regions corresponding to (D) Cu, (E) Cr, (F) Al and (G) Zn elements. All results confirmed that the anisotropic CuCrO 2 thin films were successfully prepared on the AZO by electrospinning.

Optical Properties
The CuCrO2 thin film optical transmission spectrum was examined at wavelengths of 220-1100 nm. Figure 8

Conclusions
In this study, anisotropic CuCrO2 thin films were successfully fabricated by electrospinning on glass substrate. The XRD patterns suggested that the pure phase of the Cu-CrO2 thin film was achieved by annealing in vacuum at 600 °C for 10 min. SEM revealed that higher rotation speeds resulted in better alignment of the nanowires, leading to better morphology of the CuCrO2 thin films. Moreover, higher rotation speeds also produced films with higher transmittance values. The transmittances for the different rotation speeds were 55.8% (100 rpm), 69.6% (500 rpm), 75.2% (1000 rpm), 82.1% (2000 rpm), and 81.0% (3000 rpm) in the visible region, respectively. The haze results were 16.7% (100 rpm), 15.0% (500 rpm), 12.4% (1000 rpm), 9.9% (2000 rpm), and 5.1% (3000 rpm) in the visible region, respectively. The parallel and vertical electrical conductivities of the anisotropic CuCrO2 thin films generated at 2000 rpm and 3000 rpm had differences of nearly two to three magnitude orders. In addition, higher rotation speeds yielded CuCrO2 thin films with higher optical transmittance.

Conclusions
In this study, anisotropic CuCrO 2 thin films were successfully fabricated by electrospinning on glass substrate. The XRD patterns suggested that the pure phase of the CuCrO 2 thin film was achieved by annealing in vacuum at 600 • C for 10 min. SEM revealed that higher rotation speeds resulted in better alignment of the nanowires, leading to better morphology of the CuCrO 2 thin films. Moreover, higher rotation speeds also produced films with higher transmittance values. The transmittances for the different rotation speeds were 55.8% (100 rpm), 69.6% (500 rpm), 75.2% (1000 rpm), 82.1% (2000 rpm), and 81.0% (3000 rpm) in the visible region, respectively. The haze results were 16.7% (100 rpm), 15.0% (500 rpm), 12.4% (1000 rpm), 9.9% (2000 rpm), and 5.1% (3000 rpm) in the visible region, respectively. The parallel and vertical electrical conductivities of the anisotropic CuCrO 2 thin films generated at 2000 rpm and 3000 rpm had differences of nearly two to three magnitude orders. In addition, higher rotation speeds yielded CuCrO 2 thin films with higher optical transmittance.